The perimeter is the length of the outline of a shape. To find the perimeter of a rectangle or square you have to add the lengths of all the four sides. x is in this case the length of the rectangle while y is the width of the rectangle. The perimeter, P, is: P=x+x+y+y.
I'd recomend using something like "Desmos" to plot the points so then you can see the points and the rectangle. Then count the length and width. For the this rectangle the length was 8 and the width was 7, multiply these and you get the answer of 56 sqaure units.
